The present invention relates to a photomask and, more particularly, to a photomask, which can enhance resolution of an exposure process through an improvement in depth of focus and exposure latitude margins, and a method for fabricating the same.
Generally, a semiconductor memory such as dynamic random access memory (DRAM) comprises a number of micro patterns. The micro patterns are formed through photolithography. Photolithography is generally carried out by coating a photoresist layer on a target layer of a semiconductor wafer, exposing, and developing the photoresist layer. Then, some portions of the photoresist layer are selectively removed to form photoresist layer patterns which expose some portions of the target layer. Then, the exposed portions of the target layer are removed by an etching process using the patterned photoresist layer as a mask, followed by stripping off the patterned photoresist layer from the target layer, thereby forming desired patterns on the semiconductor wafer.
However, as a pattern is further reduced via an enhanced technique, for example, to provide highly integrated semiconductor devices, manufacturers of the semiconductor devices encounter new problems in semiconductor manufacturing processes, such as an exposure process, which have not occurred in the past. In particular, reduction in size of the patterns causes problems, such as reduction in depth of focus (DOF) margin and exposure latitude (EL) margin, on the photoresist which serves to transfer the patterns to the wafer. In this regard, optical proximity correction (OPC) is performed to overcome such a problem of reduction in DOF margin and EL margin, or an assist pattern is employed to compensate for reduction in the margin of the target pattern.
A conventional method of compensating the margin of the target pattern disposed on the photomask by use of an assist pattern is described with reference to FIG. 1. Referring to FIG. 1, a conventional photomask 100 has a target pattern 120 formed on a transparent substrate 110, and an assist pattern 130 formed next to the target pattern 120. When performing an exposure process using the photomask 100, the assist pattern 130 can be transferred to the target layer, causing an undesirable pattern to be formed on a wafer. In addition, if the conventional photomask is not provided with a space in which the assist pattern 130 can be formed around the target pattern 120, the exposure process is inevitably performed without using the assist pattern 130. In other words, if some conditions for adopting the assist pattern 130 are not satisfied, the assist pattern 130 cannot be applied to every kind of micro pattern.
Thus, there are needs of a photomask and a method for fabricating the same, which can prevent the assist pattern from being formed on the wafer due to transfer of the assist pattern to the target layer, and which can enhance a process margin, for example, the DOF margin and the EL margin, in any kind of target pattern.